The present invention relates to a novel process for preparing arylpyridines from halopyridines and aryl Grignard reagents using a phosphino-palladium-ferrocene catalyst. Arylpyridines have industrial importance as precursors for active compounds in the agricultural sector.
A method frequently employed for synthesizing arylpyridines on a laboratory scale is palladium-catalyzed cross coupling in which iodoaromatics and bromoaromatics are reacted with organometallic aryl derivatives, in particular arylboronic acids or aryl Grignard reagents, in the presence of palladium or nickel catalysts. Examples describing this methodology may be found, for example, in E. Negishi, F.-T. Luo, R. Frisbee, H. Matsushita, Heterocycles 18, 1982, 117; T. Sakamoto, Y. Kondo, N. Murata, H. Yamanaka, Tetrahedron 49,1993, 9713, EP 0 834 508 A1 and WO 96/21647.
Despite the many publications in the field of synthesis of arylpyridines in the presence of nickel or palladium catalysts, no examples of a relatively large scale industrial implementation of the methods have been known up to now. This can be attributed to the fact that the catalyst systems described frequently require uneconomical amounts of catalyst or give low selectivities, i.e. high proportions of dimerization products. Moreover, large amounts of catalyst, generally &gt;1 mol %, have to be added to achieve industrially usable conversions. In addition, owing to the complexity of the reaction mixtures, no simple recycling of the catalyst is possible, so that catalyst costs generally prevent industrial implementation.
Furthermore, in the Suzuki coupling, substituted biphenyls are found when using customary catalyst systems such as Pd(Oac).sub.2 /triphenylphosphine mixtures, due to aryl transfers as seco ary reaction (D. F. O'Keefe et al., TeTrahedron Letters 1992, 6679).